1. Field of the Invention
The present invention relates to a technology for exposing a plurality of photosensitive elements to laser beams to form an image.
2. Description of the Related Art
A typical image forming apparatus, e.g., a color electrophotographic printer including a plurality of photosensitive elements (drums), uses a tandem method to form a full-color image. This type of image forming apparatus generally transfers images of different colors formed on the photosensitive elements to a recording medium such as paper in a superimposing manner to obtain a full-color image. An example of such an image forming apparatus is disclosed in Japanese Patent Application Laid-open No. 2007-163789.
FIG. 9 is an exemplary schematic diagram of relevant parts of a conventional image forming apparatus 100. FIG. 10 is a block diagram of relevant parts of the image forming apparatus 100.
The image forming apparatus 100 includes an intermediate transfer belt 2, four photosensitive elements 11K, 11Y, 11M, and 11C for black (K), yellow (Y), magenta (M), and cyan (C), respectively, and a polygon mirror 101. The intermediate transfer belt 2 is driven to rotate. The photosensitive elements 11K, 11Y, 11M, and 11C are arranged in this order on an upper surface of the intermediate transfer belt 2 along a rotating direction of the intermediate transfer belt 2. The photosensitive elements 11K, 11Y, 11M, and 11C are accompanied by charging units 12K, 12Y, 12M, and 12C and developing units 13K, 13Y, 13M, and 13C, respectively. The polygon mirror 101 deflects on its surface light beams BK, BY, BM, and BC so that the light beams impinge on the photosensitive elements 11K, 11Y, 11M, and 11C for exposure, respectively. The polygon mirror 101 includes an upper polygon mirror 101A and a lower polygon mirror 101B, each of which has six reflection side surfaces. The polygon mirror 101 is placed between a pair of fθ lenses 102. More specifically, the upper polygon mirror 101A is placed between a pair of upper fθ lenses 102A, and the lower polygon mirror 101B is placed between a pair of lower fθ lenses 102B.
Referring to FIG. 10, the image forming apparatus 100 includes a polygon-mirror control unit 32, laser-diode (LD) control units 31-1 and 31-2, scan-start-synchronization control units 34-1 and 34-2, and a scan-end-synchronization control unit 37. The polygon-mirror control unit 32 controls the polygon mirror 101. The LD control units 31-1 and 31-2 control LDs 30K, 30Y, 30M, and 30C. The scan-start-synchronization control units 34-1 and 34-2 perform synchronization control of a light beam based on detection of the light beam near a scan-start position by scan-start-synchronization control sensors 33-1 and 33-2. The scan-end-synchronization control unit 37 performs synchronization control of a light beam based on detection of the light beam near a scan-start position by a scan-end-synchronization control sensor 36.
The image forming apparatus 100 includes a reference clock generator 41, a frequency divider (M) 42, a frequency divider (N) 43, a phase-locked loop (PLL) 44, and a central processing unit (CPU) 40. The reference clock generator 41 generates a reference clock. The frequency divider (M) 42, the frequency divider (N) 43, and the PLL 44 control (change) a clock speed of the reference clock. For example, the frequency divider (M) 42, the frequency divider (N) 43, and the PLL 44 change (adjust) a pixel clock based on the reference clock received from the reference clock generator 41. The CPU 40 performs various data processing, calculation, and control operations of the image forming apparatus 100.
In the image forming apparatus 100, the polygon mirror 101 is rotated to deflect the light beams BK, BY, BM, and BC so that the light beams BK, BY, BM, and BC scan the photosensitive elements 11K, 11Y, 11M, and 11C, respectively, for exposure. Thereafter, the developing units 13K, 13Y, 13M, and 13C form (develop) toner images of K, Y, M, and C on the photosensitive elements 11K, 11Y, 11M, and 11C, respectively. The toner images are sequentially primary-transferred to the intermediate transfer belt 2 and overlaid on one another to form a full-color image. The intermediate transfer belt 2 is rotated to feed a recording medium P so that the thus-formed full-color image is secondary-transferred to the recording medium P. Hence, a full-color image is formed on the recording medium P.
Referring to FIGS. 9 and 10, in the image forming apparatus 100, the LDs 30K, 30Y, 30M, and 30C generate and emit light beams toward the reflection surfaces of the upper polygon mirror 101A and the lower polygon mirror 101B of the polygon mirror 101. The polygon mirror 101 is rotated. The direction in which the light beams impinge on the reflection surfaces of the upper polygon mirror 101A is opposite to the direction in which the light beams impinge on the reflection surfaces of the lower polygon mirror 101B. Each of the light beams is reflectively deflected by one of the six reflection surfaces of the upper polygon mirror 101A and the six reflection surfaces of the lower polygon mirror 101B. The reflectively-deflected light beam raster-scans one of the photosensitive elements 11K, 11Y, 11M, and 11C for exposure. Put another way, rotation of the polygon mirror 101 causes the light beam to sweep (hereinafter, “raster scan”) the photosensitive element in the main-scanning direction, while the rotation of the photosensitive element causes the light beam to relatively shift in the sub-scanning direction.
Because the polygon mirror 101 rotates about its rotary axis at a constant angular velocity, a scan speed of the light beam moving on the photosensitive element in the main-scanning direction for raster scan is not constant. Accordingly, when the LDs 30K, 30Y, 30M, and 30C emit at constant time intervals, resultant pixels may vary from one another in length and the like in the main-scanning direction, which leads to uneven dots. To this end, in the conventional image forming apparatus 100, the deflected light beams are subjected to fθ correction by using the fθ lenses 102 to convert the constant angular-velocity scanning into constant velocity scanning. Pixels resulting from the constant velocity scanning are identical to one another in length and the like.
As described above, in typical conventional image forming apparatuses, one or more LDs are provided for a single photosensitive element. Raster scan is performed by causing light beams, which are emitted from the LDs 30K, 30Y, 30M, and 30C, to be reflected from all the reflection surfaces that are arranged along the rotating direction of the polygon mirror 101. The raster scan and rotation of the polygon mirror are alternately repeated to expose an image. However, the conventional image forming apparatus is disadvantageous in that the number of components and a footprint for an optical system are relatively large. Therefore, the conventional image forming apparatuses can be expensive and require large footprint.
The reflection angles of the reflection surfaces of the polygon mirror 101 can fail to be identical to one another due to a dimensional error developed during the manufacturing process. Hence, it is required to correct the reflection angle of each of the reflection surfaces; that is, to perform optical-face-angle error correction. This optical-face-angle error correction is typically performed by using the fθ lens 102, which is an important element in terms of image quality as well. However, relatively-high prices of fθ lenses employed in typical image forming apparatuses have been one of the causes of relatively-high prices of the conventional image forming apparatuses. To this end, relatively-inexpensive fθ lenses made of resin can be employed; however, because resin fθ lenses are inferior to the conventionally-employed fθ lenses in temperature characteristics and optical characteristics, the resin fθ lenses can cause image quality to reduce.
As shown in FIG. 10, the image forming apparatus 100 include reflection mirrors 33M-1 and 33M-2 and the scan-start-synchronization control (detecting) sensors 33-1 and 33-2 that detect light beams for synchronization. A scan-start position of a light beam in the main-scanning direction is adjusted based on detection of the light beam by the scan-start-synchronization control sensors 33-1 and 33-2 for synchronization. However, to perform such synchronization control, it is necessary to locate the scan-start-synchronization control sensors 33-1 and 33-2 outside an image writing area so that each of the scan-start-synchronization control sensors 33-1 and 33-2 can detect a portion of a deflected light beam. Due to this arrangement, a ratio of a width of the image writing area to a stroke length (hereinafter, “raster-scan stroke”) of a light beam moving across the raster scan area in the main-scanning direction decreases. The ratio will be referred to as “effective scan ratio”. The effective scan ratio is obtained from the following equation:Effective scan ratio=(image writing area)/(raster scan area).
The effective scan ratio can also be calculated from the following equation, which is obtained from the above equation:Effective scan ratio=(number of dots to be written/write frequency)/(time duration of one turn of polygon mirror/number of reflection surfaces of polygon mirror along rotating direction).
The value of (time duration of one turn of polygon mirror/number of reflection surfaces of polygon mirror along rotating direction) depends on a resolution in the sub-scanning direction and a linear velocity (i.e., the scan speed in the main-scanning direction, hereinafter, “main-scan speed”); and the (number of dots to be written) depends on a width of the image writing area in the main-scanning direction and a resolution in the same direction. When the effective scan ratio decreases, it is required to increase the value of (write frequency) to compensate for a drop in the effective scan ratio. However, as the write frequency increases, not only does power consumption increase but also unnecessary radiation increases, which can result in build-up of cost such as running cost.
In the image forming apparatus 100, the scan-end-synchronization control (detecting) sensor 36 and a reflection mirror 36M are located near a scan-end position. Synchronization control of a light beam is performed based on detection of the light beam at the scan-end region by the scan-end-synchronization control sensor 36. Through such synchronization control, a magnification in the main-scanning direction and registration errors in the sub-scanning direction due to a temperature rise can be corrected, and an accuracy of scanning is increased. As described above, in a conventional technology, the scan-end-synchronization control sensor 36 has been provided in addition to the scan-start-synchronization control sensors 33-1 and 33-2. However, by additionally providing the scan-end-synchronization control sensor 36, a ratio of the image writing area is further decreased, unfavorably resulting in an upsizing of an optical system and upsizing of an image forming apparatus that incorporates the optical system.
To this end, the image forming apparatuses disclosed in Japanese Patent Application Laid-open No. 2003-266785, Japanese Patent Application Laid-open No. 2003-270581, and Japanese Patent Application Laid-open No. 2005-292377, include a polygon mirror having a plurality of reflection surfaces each slanting by either a first reflection angle or a second reflection angle. The reflection surfaces are arranged such that reflection surfaces having the first reflection angle and the other reflection surfaces having the second reflection angle are radially aligned in an alternating order. A light beam emitted from a light source is reflectively deflected by one of the reflection surfaces, causing the light beam to travel along one of different optical paths. The optical path is determined depending on the reflection surface. Each of a plurality of photosensitive elements is scanned with the light beam having passed through an fθ lens.
The conventional image forming apparatuses are downsized by reducing the number of light sources and the like to reduce the number of required parts as well as to reduce cost for the eliminated parts. However, the conventional image forming apparatus does not only have one or more expensive fθ lenses but also the problem pertaining to arrangement of the scan-start-synchronization control sensors 33-1, 33-2, and the scan-end-synchronization control sensor 36. Hence, there is room to further reduce the number of required parts for cost reduction and downsizing.